Accompanying the quick development of the automobile industry and more and more attention paid to environmental protection, the worldwide demand for leadless high-octane gasoline has increased continuously. In the meantime, the content of olefins and arenes contained in gasoline will also be limited. Under these circumstances, adding alkylate oil (agent) to gasoline is an effective method for maintaining a high octane number and low vapor pressure for gasoline.
Alkylate oil is a fuel product. It is a liquid product manufactured from C4 olefins and alkanes under catalysis of an acidic catalyst. It can be considered to be a special alkylation product. The C8 content and the TMP/DMH (trimethyl pentane/dimethyl hexane) ratio are important quality indexes of the alkylate oil product. Currently, the popular manufacturing methods used in industry include the sulfuric acid method and hydrofluoric acid method. In other words, concentrated sulfuric acid and hydrofluoric acid are used as catalysts to conduct the alkylation reaction between alkanes and olefins. However, these liquid strong acids are highly corrosive, which causes many problems, including difficult manufacturing process, complicated product post-treatment, and environmental pollution. Since protection of the human living environment has become a very important issue in the modern world, study and development of a new generation of alkylation catalyst and reaction technology, especially, the technology for manufacturing alkylate oil has become an important research subject in the petrochemical catalysis and reaction industrial field.
In recent years, most of the foreign and domestic studies on alkylation technology have focused on the study of solid acid catalysts and their manufacturing technology in order to solve the problems of pollution and equipment corrosion caused by the sulfuric acid method and hydrofluoric acid method. There have been many reports on various new solid catalysts used for the aforementioned alkylation reaction. However, they all have a common problem, that is, in spite of the excellent initial activity, the catalyst becomes deactivated quickly under normal conditions. The conversion rate of the olefin drops from 100% to a very low level within several hours or even tens of minutes. The main reason causing deactivation of the catalyst is the acid position of the solid acid catalyst. The olefins present in the raw material and the olefins or carbon cations generated during the reaction will undergo polymerization, cyclization, or other secondary reactions, generating C9-36 macromolecular olefin compounds, which not only cover the active sites of the catalyst but also block the pores of the catalyst. Consequently, an alkylation reaction system under supercritical conditions was developed by taking advantage of the excellent dissolving power of the supercritical fluid. However, most of the active sites are concentrated in the pores of the aforementioned catalyst, and the dissolving power of the supercritical fluid significantly decreases in the pores. Consequently, although the catalytic period of the catalyst can be prolonged under supercritical conditions, current research results indicate that a supercritical system is unable to completely prevent coking and deactivation of the catalyst. Also, since relatively high temperature and pressure are required for the supercritical reaction, the selectivity of the alkylation reaction decreases as the reaction time passes.
Relatively successful examples of solid acid catalysts in the aspect of alkylation include the supercritical system developed by the Chinese Academy of Petroleum Science and the akylene technology invented by UOP Co. of the USA. However, the aforementioned two technologies still have some disadvantages. For example, the supercritical reaction has relatively high requirements for equipment and also has the aforementioned problem, that is, the selectivity decreases as the reaction time passes. In the alkylene technology, the catalyst can be recycled for reuse. However, a large amount of solvent is needed in this technology. Recycling and reuse of these solvents will also bring many problems. In addition, both of the aforementioned two technologies require the establishment of a completely new set of manufacturing equipment, which leads to a large investment.
An ionic liquid is a salt existing in the form of a liquid at room temperature. It has many special properties. For example, its saturated vapor pressure is very low, close to zero. It can dissolve many organic and inorganic compounds and has no corrosivity. Ionic liquids with different acidities can be prepared by adjusting the types and quantities of the cations and anions. Currently, the research concerning preparation and application of ionic liquids is still growing, and increasingly more types of ionic liquids are being manufactured and application fields being developed.
There is also a relatively high number of patents concerning alkylation reactions conducted with ionic liquids used as catalyst or solvent. However, most of the patents pertain to the alkylation process of benzene and its derivatives with olefins. Examples of such patents include U.S. Pat. No. 5,994,602, U.S. Pat. No. 5,824,832, and WO 99/03163. French patent FR 2,626,572 uses imidazolium or quaternary amine ionic liquids as a catalyst for isoalkanes and olefins. However, the main reaction products of this patent are isomeric C6 or C7, while the content of C8 alkanes in the product is very low. It does not have practical application value for alkylate oil. Also, the imidazolium ionic liquids are not well used or popularized because of their difficult synthesis process and high price.
Both U.S. Pat. No. 5,731,101 and WO 00/41809 provide a method that can easily manufacture ionic liquids at room temperature. In other words, the hydrohalide of an alkyl-containing amine reacts with a metal halide to manufacture an ionic liquid. This product is in the form of a liquid at room temperature. Its anionic part only contains one metal. The aforementioned patents in this field also disclose applications of this type of ionic liquid as a catalyst for alkylation of benzene and olefins but has no description pertaining to the alkylation reaction between isoalkanes and olefins, that is, manufacturing of alkylate oil.
It is well known to the researcher in this field that one of the keys for manufacturing high-quality alkylate oil is to increase the selectivity of the alkylation reaction. None of the aforementioned conventional alkylation reaction methods can satisfy this demand.
The Stratoc reactor that is usually used during the process of manufacturing alkylate oil using the aforementioned industrial sulfuric acid method has a complicated configuration, the equipment is difficult to repair and maintain, and it requires a large investment. The reactor used in the hydrofluoric acid method has a relatively simple configuration. However, since it uses many types of auxiliary facilities for preventing leakage of hydrofluoric acid, the equipment and process are also complicated. In addition, both of the liquid acids are highly corrosive and have high requirements for the materials of the equipment. Also, the alkane/olefin ratio in the reactor required in the alkylation process carried out using the sulfuric acid method and the hydrofluoric acid method must reach several hundred or even a thousand in order to guarantee the high quality of the product. This requires the circulation of a large amount of isobutane, which significantly increases the operation load of the fractionating column, thus increasing the operation cost. Or, part of the product can be recycled to maintain a relatively high alkane/olefin ratio in order to reduce the amount of the recycled isobutane. This, however, increases the contact time of the alkylate oil product and the catalyst. As a result, decomposition and/or polymerization of the alkylate oil is increased, which lowers the selectivity of the alkylate oil product.
In summary, there are many disadvantages to be overcome or to improve in the current technologies for the preparation or manufacturing of alkylate oil products.